Personal communication networks are being deployed extensively world-wide using cellular mobile radio systems. Earlier networks, still in operation, use analogue modulation formats for the radio air interface protocol. These analogue networks exhibit the problem of call saturation in high usage areas. To overcome this problem higher capacity air interface protocols using digital modulation format networks have been introduced in tandem, that is an area is covered by both systems.
In the United States and Canada the early standardised analogue network known as AMPS has reached a fairly universal coverage of the populated North American continent. The newer digital networks, however, tend to be deployed in areas of high usage. A result of this is that there are areas of digital network coverage overlaying a universal analogue network coverage. Additionally, different air interface protocol standards of digital networks have been deployed regionally, since different telecommunications operators have developed their own protocols or have developed such protocols in line with national and sometimes international standards authorities, for example, the GSM protocol. Whilst it is reasonable to suppose that handsets operable for different radio communications protocols are similar from the users point of view, it is not possible, in particular, to use a digital mobile radio in an analogue cellular region and vice versa. This stems from the fact that whilst both types of handsets possess antennas, radio front end transmitter, receiver and baseband circuits, they operate on different air interface protocols which operate, inter alia at different radio carrier frequencies.
Therefore it can be seen that each individual personal communications system user will need a dual network service for complete coverage. Consequently the user requires a handset that will not only function throughout the coverage area of the specific subscribed-to digital network, but also have a switched alternative mode to operate on the universal analogue network.
The problem of implementing a dual mode handset has been considered to be surmountable by several different approaches; one solution uses two separate radio transceivers piggybacked and combined at the man-machine interface (keyboard and audio); a second solution uses two separate radio sections piggybacked and combined at the digital signal processing part of the radio transceiver, --applicants have a pending application relating to such a scheme, GB9603316.2. These two above approaches have problems in that both modes of operation transmit via an antenna. If the frequencies of operation are different, as indeed they will need to be, then two types of antenna will be necessary.
For a dual mode terminal perhaps the simplest option for the antenna is to use a separate element for each of the desired bands. This could be in form of one external and one internal antenna, or two internal antennas. Two external antennas would be cumbersome, and unsightly. Such use of two antennas which have separate resonant frequencies of operation is accordingly complicated and unwieldy.
For the case where one external and one internal antenna is used it may be better to use the internal antenna to serve the higher frequency band. This keeps the size of the internal antenna down, thus ensuring that the volume required for the antenna inside the handset is kept to a minimum. For the external antenna a standard extractable monopole could be used, with a helix on the end for when the antenna is retracted. Alternatively, a fixed external antenna could be used. For the internal antenna a bent folded monopole could be employed. However, bent folded monopole elements do not provide sufficient bandwidth; for a typical, efficient bent folded monopole element one night expect a 5-7% 10 dB return loss bandwidth.
A number of dual band helical structures have been investigated at the Helsinki University of Technology, and these were presented at the 1996 IEEE VTC Conference. The helical structures presented are shown in FIG. 1. They consist of: (a) two helical antennas, one within the other; (b) a helical-monopole combination; and (c) a helical antenna combined with a wound monopole. The paper states that the dual frequency operation can be obtained from all three of the structures that are shown. Results for structure (a) state that it was tuned to the frequencies 1740 MHz and 900 MHz, and that 10 dB return loss bandwidths were obtained of 5.2% and 2.2% respectively. The dimensions for the antenna were D.sub.1 =6 mm, lh1-12 mm, N.sub.1 =5, D.sub.2 -3 mm, l.sub.h2 =14 mm, N.sub.2 =5, and 1.sub.s =10 mm. Results for structure (b) state that it was tuned to the frequencies 1750 MHz and 894 MHz, and that 10 dB return loss bandwidths were obtained of 12% and 4.5% respectively. Structure (c) is simply a more compact version of (b), and not surprisingly has a narrower bandwidth. For the upper and lower bands, measured bandwidths of 11% and 2.9% were obtained where the overall structure height was 34 mm. Thus, in summary these antennas provide a bandwidth which is not sufficient for many radio applications, and also does not leave any margin for manufacturing tolerances.
A dual band external antenna is described by Ali et al in `A wide band dual meander sleeve antenna`, IEEE Antennas and Propagation Society International Symposium, 1995, vol.2 p.1124-7, Jun. 18-23, 1995, Newport Beach, Calif., USA, and this is called the wide band dual meander sleeve antenna. This antenna is described as potentially useful as a low profile antenna for a dual mode handset. However, the results presented in the paper are for the case where the experimental antenna is mounted on a large ground plane (90 cm.sup.2).
One possible configuration for a combination of antenna elements could be a monopole used to serve the AMPs radio, and an internal bent folded monopole used to serve the PCS radio. Disadvantages arising from such a configuration are: the SAR performance may be unacceptable; the monopole is susceptible to damage/breakage; and, Isolation may be poor.
A still further option is the use of a single antenna structure which is combined with a dual band matching network, but this is also complicated and unwieldy.